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<B><A HREF="FCNTL.html">FCNTL(2)</A></B>		  FreeBSD System Calls Manual		      <B><A HREF="FCNTL.html">FCNTL(2)</A></B>


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<H2>NAME</H2><PRE>
     <B>fcntl</B> - file control


</PRE>
<H2>SYNOPSIS</H2><PRE>
     <B>#include</B> <B>&lt;fcntl.h&gt;</B>

     <I>int</I>
     <B>fcntl</B>(<I>int</I> <I>fd</I>, <I>int</I> <I>cmd</I>, <I>...</I>)


</PRE>
<H2>DESCRIPTION</H2><PRE>
     <B>Fcntl</B>() provides for control over descriptors.  The argument <I>fd</I> is a de-
     scriptor to be operated on by <I>cmd</I> as described below.  Depending on the
     value of <I>cmd</I>, <B>fcntl</B> can take an additional third argument <I>int</I> <I>arg</I>.

     F_DUPFD	Return a new descriptor as follows:

		    <B>o</B>	Lowest numbered available descriptor greater than or
			equal to <I>arg</I>.
		    <B>o</B>	Same object references as the original descriptor.
		    <B>o</B>	New descriptor shares the same file offset if the ob-
			ject was a file.
		    <B>o</B>	Same access mode (read, write or read/write).
		    <B>o</B>	Same file status flags (i.e., both file descriptors
			share the same file status flags).
		    <B>o</B>	The close-on-exec flag associated with the new file
			descriptor is set to remain open across <B><A HREF="execve.html">execve(2)</A></B> sys-
			tem calls.

     F_GETFD	Get the close-on-exec flag associated with the file descriptor
		<I>fd</I>. If the low-order bit of the returned value is 0, the file
		will remain open across <B>exec</B>(), otherwise the file will be
		closed upon execution of <B>exec</B>() (<I>arg</I> is ignored).

     F_SETFD	Set the close-on-exec flag associated with <I>fd</I> to the low order
		bit of <I>arg</I> (0 or 1 as above).

     F_GETFL	Get descriptor status flags, as described below (<I>arg</I> is ig-
		nored).

     F_SETFL	Set descriptor status flags to <I>arg</I>.

     F_GETOWN	Get the process ID or process group currently receiving SIGIO
		and SIGURG signals; process groups are returned as negative
		values (<I>arg</I> is ignored).

     F_SETOWN	Set the process or process group to receive SIGIO and SIGURG
		signals; process groups are specified by supplying <I>arg</I> as neg-
		ative, otherwise <I>arg</I> is interpreted as a process ID.

     The flags for the F_GETFL and F_SETFL flags are as follows:

     O_NONBLOCK   Non-blocking I/O; if no data is available to a <B><A HREF="read.html">read(2)</A></B> call,
		  or if a <B><A HREF="write.html">write(2)</A></B> operation would block, the read or write
		  call returns -1 with the error EAGAIN.

     O_APPEND	  Force each write to append at the end of file; corresponds
		  to the O_APPEND flag of <B><A HREF="open.html">open(2)</A></B>.

     O_ASYNC	  Enable the SIGIO signal to be sent to the process group when
		  I/O is possible, e.g., upon availability of data to be read.

     Several commands are available for doing advisory file locking; they all
     operate on the following structure:

     struct flock {
	     off_t   l_start;	     /* starting offset */
	     off_t   l_len;	     /* len = 0 means until end of file */
	     pid_t   l_pid;	     /* lock owner */
	     short   l_type;	     /* lock type: read/write, etc. */
	     short   l_whence;	     /* type of l_start */
     };
     The commands available for advisory record locking are as follows:

     F_GETLK	Get the first lock that blocks the lock description pointed to
		by the third argument, <I>arg</I>, taken as a pointer to a <I>struct</I>
		<I>flock</I> (see above).  The information retrieved overwrites the
		information passed to <B>fcntl</B>() in the <I>flock</I> structure.  If no
		lock is found that would prevent this lock from being created,
		the structure is left unchanged by this function call except
		for the lock type which is set to F_UNLCK.

     F_SETLK	Set or clear a file segment lock according to the lock de-
		scription pointed to by the third argument, <I>arg</I>, taken as a
		pointer to a <I>struct</I> <I>flock</I> (see above).	F_SETLK is used to es-
		tablish shared (or read) locks (F_RDLCK) or exclusive (or
		write) locks, (F_WRLCK), as well as remove either type of lock
		(F_UNLCK). If a shared or exclusive lock cannot be set,
		<B>fcntl</B>() returns immediately with EAGAIN.

     F_SETLKW	This command is the same as F_SETLK except that if a shared or
		exclusive lock is blocked by other locks, the process waits
		until the request can be satisfied.  If a signal that is to be
		caught is received while <B>fcntl</B>() is waiting for a region, the
		<B>fcntl</B>() will be interrupted if the signal handler has not
		specified the SA_RESTART (see <B><A HREF="sigaction.html">sigaction(2)</A></B>).

     When a shared lock has been set on a segment of a file, other processes
     can set shared locks on that segment or a portion of it.  A shared lock
     prevents any other process from setting an exclusive lock on any portion
     of the protected area.  A request for a shared lock fails if the file de-
     scriptor was not opened with read access.

     An exclusive lock prevents any other process from setting a shared lock
     or an exclusive lock on any portion of the protected area.  A request for
     an exclusive lock fails if the file was not opened with write access.

     The value of <I>l</I><B>_</B><I>whence</I> is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that
     the relative offset, <I>l</I><B>_</B><I>start</I> bytes, will be measured from the start of
     the file, current position, or end of the file, respectively.  The value
     of <I>l</I><B>_</B><I>len</I> is the number of consecutive bytes to be locked.	If <I>l</I><B>_</B><I>len</I> is
     negative, the result is undefined.  The <I>l</I><B>_</B><I>pid</I> field is only used with
     F_GETLK to return the process ID of the process holding a blocking lock.
     After a successful F_GETLK request, the value of <I>l</I><B>_</B><I>whence</I> is SEEK_SET.

     Locks may start and extend beyond the current end of a file, but may not
     start or extend before the beginning of the file.	A lock is set to ex-
     tend to the largest possible value of the file offset for that file if
     <I>l</I><B>_</B><I>len</I> is set to zero. If <I>l</I><B>_</B><I>whence</I> and <I>l</I><B>_</B><I>start</I> point to the beginning of
     the file, and <I>l</I><B>_</B><I>len</I> is zero, the entire file is locked.  If an applica-
     tion wishes only to do entire file locking, the <B><A HREF="flock.html">flock(2)</A></B> system call is
     much more efficient.

     There is at most one type of lock set for each byte in the file.  Before
     a successful return from an F_SETLK or an F_SETLKW request when the call-
     ing process has previously existing locks on bytes in the region speci-
     fied by the request, the previous lock type for each byte in the speci-
     fied region is replaced by the new lock type.  As specified above under
     the descriptions of shared locks and exclusive locks, an F_SETLK or an
     F_SETLKW request fails or blocks respectively when another process has
     existing locks on bytes in the specified region and the type of any of
     those locks conflicts with the type specified in the request.

     This interface follows the completely stupid semantics of System V and
     IEEE Std1003.1-1988 (``POSIX'') that require that all locks associated
     with a file for a given process are removed when <I>any</I> file descriptor for
     that file is closed by that process.  This semantic means that applica-
     tions must be aware of any files that a subroutine library may access.
     For example if an application for updating the password file locks the
     password file database while making the update, and then calls getpw-
     <B><A HREF="nam.html">nam(3)</A></B> to retrieve a record, the lock will be lost because <B><A HREF="getpwnam.html">getpwnam(3)</A></B>
     opens, reads, and closes the password database.  The database close will
     release all locks that the process has associated with the database, even
     if the library routine never requested a lock on the database.  Another
     minor semantic problem with this interface is that locks are not inherit-
     ed by a child process created using the <B><A HREF="fork.html">fork(2)</A></B> function.	The <B><A HREF="flock.html">flock(2)</A></B>
     interface has much more rational last close semantics and allows locks to
     be inherited by child processes.  <B><A HREF="Flock.html">Flock(2)</A></B> is recommended for applica-
     tions that want to ensure the integrity of their locks when using library
     routines or wish to pass locks to their children.	Note that <B><A HREF="flock.html">flock(2)</A></B> and
     <B><A HREF="fcntl.html">fcntl(2)</A></B> locks may be safely used concurrently.

     All locks associated with a file for a given process are removed when the
     process terminates.

     A potential for deadlock occurs if a process controlling a locked region
     is put to sleep by attempting to lock the locked region of another pro-
     cess.  This implementation detects that sleeping until a locked region is
     unlocked would cause a deadlock and fails with an EDEADLK error.


</PRE>
<H2>IMPLEMENTATION NOTES</H2><PRE>
     In the non-threaded library <B>fcntl</B>() is implemented as the <I>fcntl</I> syscall.

     In the threaded library, the <I>fcntl</I> syscall is assembled to
     <B>_thread_sys_fcntl</B>() and <B>fcntl</B>() is implemented as a function which dis-
     ables thread rescheduling, locks <I>fd</I> for read and write, then calls
     <B>_thread_sys_fcntl</B>().  Before returning, <B>fcntl</B>() unlocks <I>fd</I> and enables
     thread rescheduling.


</PRE>
<H2>RETURN VALUES</H2><PRE>
     Upon successful completion, the value returned depends on <I>cmd</I> as follows:

	   F_DUPFD    A new file descriptor.

	   F_GETFD    Value of flag (only the low-order bit is defined).

	   F_GETFL    Value of flags.

	   F_GETOWN   Value of file descriptor owner.

	   other      Value other than -1.

     Otherwise, a value of -1 is returned and <I>errno</I> is set to indicate the er-
     ror.


</PRE>
<H2>ERRORS</H2><PRE>
     <B>Fcntl</B>() will fail if:

     [EAGAIN]	   The argument <I>cmd</I> is F_SETLK, the type of lock <I>(l</I><B>_</B><I>type)</I> is a
		   shared lock (F_RDLCK) or exclusive lock (F_WRLCK), and the
		   segment of a file to be locked is already exclusive-locked
		   by another process; or the type is an exclusive lock and
		   some portion of the segment of a file to be locked is al-

		   ready shared-locked or exclusive-locked by another process.

     [EBADF]	   <I>Fildes</I> is not a valid open file descriptor.

		   The argument <I>cmd</I> is F_SETLK or F_SETLKW, the type of lock
		   <I>(l</I><B>_</B><I>type)</I> is a shared lock (F_RDLCK), and <I>fildes</I> is not a
		   valid file descriptor open for reading.

		   The argument <I>cmd</I> is F_SETLK or F_SETLKW, the type of lock
		   <I>(l</I><B>_</B><I>type)</I> is an exclusive lock (F_WRLCK), and <I>fildes</I> is not
		   a valid file descriptor open for writing.

     [EDEADLK]	   The argument <I>cmd</I> is F_SETLKW, and a deadlock condition was
		   detected.

     [EINTR]	   The argument <I>cmd</I> is F_SETLKW, and the function was inter-
		   rupted by a signal.

     [EINVAL]	   <I>Cmd</I> is F_DUPFD and <I>arg</I> is negative or greater than the max-
		   imum allowable number (see <B><A HREF="getdtablesize.html">getdtablesize(2)</A></B>).

		   The argument <I>cmd</I> is F_GETLK, F_SETLK, or F_SETLKW and the
		   data to which <I>arg</I> points is not valid, or <I>fildes</I> refers to
		   a file that does not support locking.

     [EMFILE]	   The argument <I>cmd</I> is F_DUPFD and the maximum number of file
		   descriptors permitted for the process are already in use,
		   or no file descriptors greater than or equal to <I>arg</I> are
		   available.

     [ENOLCK]	   The argument <I>cmd</I> is F_SETLK or F_SETLKW, and satisfying the
		   lock or unlock request would result in the number of locked
		   regions in the system exceeding a system-imposed limit.

     [EPERM]	   <I>Cmd</I> is F_SETOWN and the process ID or process group given
		   as an argument is in a different session than the caller.

     [ESRCH]	   <I>Cmd</I> is F_SETOWN and the process ID given as argument is not
		   in use.

     In addition, if <I>fd</I> refers to a descriptor open on a terminal device (as
     opposed to a descriptor open on a socket), a <I>cmd</I> of F_SETOWN can fail for
     the same reasons as in <B><A HREF="tcsetpgrp.html">tcsetpgrp(3)</A></B>,  and a <I>cmd</I> of F_GETOWN for the rea-
     sons as stated in <B><A HREF="tcgetpgrp.html">tcgetpgrp(3)</A></B>.


</PRE>
<H2>SEE ALSO</H2><PRE>
     <B><A HREF="close.html">close(2)</A></B>,	<B><A HREF="execve.html">execve(2)</A></B>,  <B><A HREF="flock.html">flock(2)</A></B>,  <B><A HREF="getdtablesize.html">getdtablesize(2)</A></B>,  <B><A HREF="open.html">open(2)</A></B>,  <B><A HREF="sigvec.html">sigvec(2)</A></B>,
      <B><A HREF="tcgetpgrp.html">tcgetpgrp(3)</A></B>,  <B><A HREF="tcsetpgrp.html">tcsetpgrp(3)</A></B>


</PRE>
<H2>HISTORY</H2><PRE>
     The <B>fcntl</B>() function call appeared in 4.2BSD.

4.2 Berkeley Distribution      January 12, 1994 			     4
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